STONY BROOK (US) — Biomass burning in the Southern Hemisphere over the past 650 years appears to be the cause of a fluctuating pattern of carbon monoxide in the Earth’s atmosphere.

Scientists studying Antarctic ice cores found that traces of carbon monoxide isotopes in the ice samples show that the amount of biomass burning—fires fueled by plant materials such as wood, peat, and grasses resulting from natural forest fires and man-made cooking and communal fires—did not gradually rise over time, but rather rose and fell across the centuries.

“It was a big surprise,” says John Mak, associate professor in the school of marine and atmospheric sciences at Stony Brook University. “We certainly weren’t looking for that. It’s kind of a mystery.”

The study is published in the early online version of the journal Science.

While the biomass burning trends were similar to those found in previous studies that measured other tracers of biomass burning, this was the first study to measure variations in stable isotopes of carbon and oxygen, the first such measurements for carbon monoxide collected from ice core samples.

The results indicate “that large variations in the degree of biomass burning in the Southern Hemisphere occurred during the last 650 years, with a decrease by about 50 percent in the 1600s, an increase of about 100 percent by the late 1800s, and another decrease by about 70 percent from the late 1800s to present day.”

The evidence suggests that there was less biomass burning in the 20th century than in the century to century-and-a-half preceding it.

“One might think there should clearly have been more biomass burning in the Southern Hemisphere during the Industrial and post-Industrial eras. It seems logical to conclude that,” Mak says.

However, both the CO measurement study and previous studies that measured charcoal particles in sediment and methane in trapped ice in the Southern Hemisphere have pointed the other way. The research team studied two ice cores from two different locations in Antarctica.

A natural follow-up to the study will be “to extend the record further back in time in Antarctica, and also we clearly want to do a similar type of study in the Northern Hemisphere from ice cores in Greenland,” Mak says.

“That would be more difficult in the Northern Hemisphere because of the potential for complications in the Northern Hemisphere that we don’t have in the Southern Hemisphere. There’s more going on in the Northern Hemisphere.”